Jailbreaking is a process that allows iPad, iPhone and iPod Touch users to run third-party unsigned code on their devices by unlocking the operating system and allowing the user root access. Once jailbroken, iPhone users are able to download many extensions and themes previously unavailable through the App Store via unofficial installers such as Cydia. A jailbroken iPad, iPhone or iPod Touch is still able to use the App Store and iTunes.

Jailbreaking is different from SIM unlocking, which, once completed, means that the mobile phone will accept any SIM without restriction on, for example, the country or network operator of origin. Jailbreaking, according to Apple, can void Apple's warranty on the device, although this is quickly remedied by restoring the device in iTunes.

Cydia founder Jay Freeman estimates that over 10% of all iPhones are jailbroken.

The first jailbreaking method was released on July 10, 2007, and a month later the first third-party game was released for the iPhone and iPod Touch.
Three months after the initial jailbreak, another method was discovered, which led to a cat and mouse game between Apple and hackers to patch and exploit security holes.

In February the Italian hacker Zibri found the most important key inside the iPhone which then lead to all upcoming jailbreak methods. Zibri himself coded and freely distributed ZiPhone, the easiest jailbreak of all. Upon the nearing release of OS 2.0 (previously iPhone OS), a hacker group called the iPhone Dev Team released a jailbreaking application named PwnageTool that used a graphical user interface to jailbreak 2G and 3G versions of the iPhone, and the first generation of iPod Touches then available. In January 2009, the iPhone Dev Team commenced work on jailbreaking the iPod Touch 2G. On March 17, 2009, Apple announced the release of iOS 3.0, which remained unbroken until the iPhone Dev Team released PwnageTool 3.0 and redsn0w 0.7.2 three months later. The iOS 3.1 release in September 2009 again disabled jailbreaking, which was followed by a new version of PwnageTool able to jailbreak all devices except the iPod Touch 3G; the iPod Touch would remain unhacked for a month, until the release of George Hotz's blackra1n.

redsn0w : http://redsn0w.com/
Works for: Redsnow supports all devices on ALL iOS VERSION, EXCEPT for iPad 2 and iPhone 4S at this time
Tethered: Only for firmware > 5.0.1 (For now, as of version 0.9.10b3)

PwnageTool: http://redsn0w.com/
Works for: Same as the latest version of redsn0w PwnageTool is for MAC only!
Tethered: Same as the latest version of redsn0w

sn0wbreeze 2
Works for: All devices that supports the old "24K Bootrom Exploit" (Launch iPhone 3GS with old bootrom, all iPhone 3G/original, iPod Touch 2G/1G with model MBXXXX). Current version of sn0wbreeze supports up to firmware 4.1
Similar to the older redsn0w, except sn0wbreeze allows you to build a custom ipsw restore file with more options, such as injecting .deb packages. May need to work with iReb collaboratively.
Tethered: Same as above.

Predecessors:

JailbreakMe:http://www.jailbreakme.com/ (Visit this site on Mobile Safari)
Works for: 4.0 - 4.3.3 (All iDevices including iPad 2)
Tethered: No

greenpois0n: http://www.greenpois0n.com/
Originally based on SHAtter, but is now using the USB Bootrom which Limera1n is based off of. Giving up two valuable bootrom exploits in one go is not worth it.
Works on 4.0 and ALL newer firmwares released in the futuregreenpois0n is recommended over limera1n as it is more reliable, and more stable. Not to mention that it needs less hassle to jailbreak the device.
Tethered: Only for firmware > 4.2.1

limera1n: http://www.limera1n.com/
A bootrom based jailbreak tool that is fors for ALL DEVICES at the moment, until Apple releases a new line of hardware revisions to patch this.
Works on 4.0 and ALL newer firmwares released in the future
Tethered: Only for firmware > 4.1

Note: If you are below the firmware version 3.1.2 then it is suggested that you upgrade to the latest jailbreakable firmware.
For iPhone original and iPod Touch original, the latest firmware is 3.1.3, and can be jailbroken by Spirit or Jailbreakme.com
For iPhone 3G, iPod Touch 2G, iPhone 3GS, iPhone 4, iPod Touch 3G, and iPod Touch 4G with the current unpatched bootrom, all firmwares can be jailbroken using redsn0w.

Q:What is the differences between a "tethered" jailbreak and a "untethered" jailbreak?
A:Think of a tetherball: It's a ball tied to a pole by a chain. Tethered jailbreak means that the device will go to the "connect to iTunes" screen every time it was rebooted, and requires you to run the jailbreak tool again. Untethered means that the process is one-time. You can restart your device all you want, and it will boot fine.

Q:If I update my jailbroken iDevice to the latest firmware for jailbreak, will I lose all of my data?
A: Unless you selected "Restore", then no, your songs, videos, photos, notes, appstore apps, and all other "official" content will not be erased. However, ALL CYDIA APPS WILL BE REMOVED.

Q: OMFG If I screw up, will my device turn into a brick permanently?? :ohdear:
A: No. You can always restore your device by placing it into DFU mode.

Q: Can I downgrade my firmware to a previous version?
A: For iPod Touch 2G, iPhone 3G, and devices that are older, you can. For newer devices, they cannot be downgraded unless you have a "SHSH blob" for the specific version you are attempting to downgrade saved. More information regarding on this topic will be added later.

Q: When I use redsn0w, the jailbreak process gets stuck at "Uploading ramdisk"
A: Run redsn0w as administrator, and if necessary, under Windows XP SP3 compatibility mode.

Q: I don't like SSH. How can I get files onto my device without it?
A: The easiest way would be downloading Total Commander and an addon for it called "T-Pot". This allows you to upload files to your iDevice to anywhere, including the root directories if your device is jailbroken.

Q: When I tries to jailbreak my device, it didn't work/I got an error/how to solve this problem?
A: Be descriptive, we cannot read your mind over tcp/ip (or at all)

:siren:STOP POSTING SCREENSHOTS OF YOUR HOMESCREEN, LOCKSCREENS, AND ETC:siren:
There is a designated thread for it, so use it. No one in this thread cares about how you pimped out your device; they only clutter up the thread. If you would like to tell us that you were successful with your jailbreak process, that's great, go for it. Attaching a photo of your homescreen is not necessary.

So is it really a permanent thing that makes the phone 100% un-unlockable using limera1n?

Unless new "exploits" can be found within the new basebands firmware, and the new firmware is automatically installed with the new official update, and it cannot be downgraded. At the moment, only the original baseband firmware found in >=4.0.2 can be used for the ultrasn0w unlock

The iOS 4.1.1 update is expected to be released soon by Apple. According to ZDNet, the current version of the iOS 4.1 software has a daylight savings bug which Apple says it will fix and release in an upcoming update.
The limera1n jailbreak was recently released, causing the greenpois0n jailbreak with SHAtter exploit to be delayed. Apple may be patching up the “untethered” part of the exploit used in limera1n once iOS 4.1.1 comes out, so it’s good that greenpois0n SHAtter exploit was saved. If you are using limera1n, remember to stay away from iOS 4.1.1 update as it’s just a minor update.

Update: By patched I mean that the untethered part of the jailbreak can be patched. You can still jailbreak your device and have it tethered, but that would be really annoying because you’d need to connect your device to your computer every time you reboot it.

Well, looks like we lose the untethered side of this jailbreak if we update.

I just unlocked my 3gs the legit way, which required me to update to 4.1 from 3.1.2. Fucking required.
Wondering if I should use Greenpois0n or Limera1n this time around. Seems like people have a lot of issues, so I'll be waiting a tad.

So I used greenpoison. What does one do if the jailbreak worked, but the cydia install fails?http://img251.imageshack.us/img251/3180/photods.png
Direct link, can't get image tags working
First it was just "loader" that was white. No cydia icon. Then I opened loader, it installed cydia but crashed halfway through, atleast I think so, didn't look at screen. It appeared after restarting the phone as white, and won't launch. Loader can't remove cydia, and it won't reinstall it.
What can I do?

So I used greenpoison. What does one do if the jailbreak worked, but the cydia install fails?
First it was just "loader" that was white. No cydia icon. Then I opened loader, it installed cydia but crashed halfway through, atleast I think so, didn't look at screen. It appeared after restarting the phone as white, and won't launch. Loader can't remove cydia, and it won't reinstall it.
What can I do?

Mine showed some "warnings" that's all I think, and a few not found. Restored, rejailbreaked and used the loader without my SIM being unlocked worked, so had to use wireless internet. I guess I recieved an SMS while loader installed Cydia the first time, which crashed Loader and disrupted the installation. Cydia works, thanks psp!